Mirror Socket, Optical System and Projection Exposure Apparatus

The present application is a continuation of, and claims benefit under 35 USC 120 to, international application No. PCT/EP2023/08364, filed Nov. 30, 2023, which claims benefit under 35 USC 119 of Greek Application No. 20220101024, filed Dec. 9, 2022, and German Application No. 10 2023 100 393.3, filed Jan. 10, 2023. The entire disclosure of each of these applications is incorporated by reference herein.

The present disclosure relates to a mirror socket for an optical element, to an optical system having such a mirror socket and to a projection exposure apparatus having such a mirror socket and/or such an optical system.

Microlithography is used for the production of microstructured components, for example integrated circuits. The microlithography process is carried out using a lithography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is projected here via the projection system onto a substrate, for example a silicon wafer, which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate. Driven by the desire for ever smaller structures in the production of integrated circuits, EUV lithography apparatuses that use light at a wavelength ranging from 0.1 nanometer (nm) to 30 nm, for example 13.5 nm, are currently under development. In the case of such EUV lithography apparatuses, because of the high absorption of light at this wavelength by most materials, reflective optical units, which is to say mirrors, are typically used instead of—as previously—refractive optical units, which is to say lens elements.

With the aid of mirror sockets, a mirror as mentioned above of a projection system may be coupled to a support structure, for example in the form of a force frame, or to actuators for aligning the mirror. To this end, the mirror sockets are often adhesively bonded to the mirror. The mirror has six degrees of freedom, specifically three translational degrees of freedom along a first spatial direction, a second spatial direction and a third spatial direction, and three rotational degrees of freedom, in each case about the aforementioned spatial directions.

Exactly three mirror sockets are often provided, with each mirror socket being assigned exactly two degrees of freedom. However, this is not mandatory. By way of example, it is also possible for one mirror socket to be assigned three degrees of freedom, a further mirror socket to be assigned two degrees of freedom and a further mirror socket to be assigned one degree of freedom. To introduce as few forces as possible into the mirror, as this may lead to unwanted deformations of the mirror for example, it is desirable for the mirror sockets to have stiffnesses of different magnitudes in different spatial directions.

The present disclosure seeks to provide an improved mirror socket for an optical element. A mirror socket for an optical element is proposed. The mirror socket comprises a centre axis, a first spatial direction oriented perpendicular to the centre axis and a second spatial direction oriented perpendicular to the centre axis and perpendicular to the first spatial direction, wherein the mirror socket has a first stiffness viewed in the first spatial direction and a second stiffness viewed in the second spatial direction and wherein the first stiffness and the second stiffness have different magnitudes.

As a result of the mirror socket having stiffnesses of different magnitudes viewed in the first spatial direction and in the second spatial direction, it is possible to arrange a plurality of mirror sockets carrying the optical element in such a way that no unwanted forces or moments are introduced into the optical element. Unwanted deformations of an optically effective surface of the optical element can be avoided as a result. This can improve the imaging quality of a projection exposure apparatus having such a mirror socket.

The optical element can be a mirror module or a mirror, such as an EUV mirror, or can be referred to as mirror module or mirror. However, the optical element can also be a lens element. Optionally, a plurality of mirror sockets, for example exactly three, are assigned to the optical element. Only one mirror socket is discussed in more detail hereinafter. The optical element has an optically effective surface, for example a mirror surface. The optical element, and especially the optically effective surface, is suitable for reflecting illumination radiation, for example EUV radiation. The optically effective surface can be a coating that is applied to a substrate, for example a glass block or a glass ceramic block. Optionally, the mirror socket can be integrally bonded to the optical element, for example to a back side of the optical element. Integrally bonded connections are connections in which the connection partners are held together by atomic or molecular forces. At the same time, they are non-releasable connections that can be separated only by destruction of the connection mechanism and/or the connection partners. By way of example, the mirror socket is adhesively bonded to the optical element.

The mirror socket can be constructed in rotationally symmetric fashion with respect to the centre axis. Optionally, the mirror socket is constructed in substantially rotationally symmetric fashion with respect to the centre axis. In this case “substantially” means that it is not possible to rule out that the mirror socket also has regions or portions which are not constructed in rotationally symmetric fashion with respect to the centre axis. The centre axis may also be referred to as the axis of symmetry of the mirror socket. For example, at least an outer part and/or an inner part of the mirror socket are constructed in rotationally symmetric fashion with respect to the centre axis. However, the rotational symmetry is not mandatory.

Each mirror socket can be assigned a coordinate system having the first spatial direction, which may also be referred to as x-direction, the second spatial direction, which may also be referred to as y-direction, and a third spatial direction or z-direction. The third spatial direction may be oriented parallel to the centre axis or may correspond to the latter. The second spatial direction is oriented perpendicular to the first spatial direction. The third spatial direction is oriented perpendicular to the first spatial direction and perpendicular to the second spatial direction. In the present case, “perpendicular” should be understood to mean an angle of 90°±10°, such as of 90°±5°, for example of 90°±3°, for example of 90°±1°, and for example of exactly 90°. The first spatial direction and the second spatial direction span a plane oriented perpendicular to the centre axis.

In the present case, the “stiffness” should be understood to mean very generally the resistance of a body, in the present case the mirror socket, to an elastic deformation impressed thereon by an external load and conveys the relationship between the load on the body and its deformation. The stiffness is determined by the material in the body and its geometry. For example, the first stiffness and the second stiffness having different magnitudes can be achieved by adapting a geometry of the mirror socket. Optionally, the second stiffness is greater than the first stiffness.

For example, the mirror socket has a third stiffness viewed along the centre axis or in the third spatial direction. The third stiffness can be greater than the first stiffness and greater than the second stiffness. The first stiffness and the second stiffness having “different” magnitudes in the present case means that, in particular, the second stiffness is greater than the first stiffness. Alternatively, the first stiffness can also be greater than the second stiffness.

According to an embodiment, the mirror socket further comprises an outer part, an inner part arranged within the outer part and elastically deformable connecting parts, with the outer part being connected to the inner part with the aid of the connecting parts.

The outer part can be ring shaped. Therefore, the outer part may also be referred to as outer ring. The outer part can be constructed in rotationally symmetric fashion with respect to the centre axis. The inner part can be ring shaped. Therefore, the inner part may also be referred to as inner ring. The inner part can be constructed in rotationally symmetric fashion with respect to the centre axis. Optionally, the outer part is connected to the optical element, for example adhesively bonded thereto. The inner part can be connected to a support structure, for example in the form of a force frame. The inner part serves as an interface to surroundings. In the present case, the “surroundings” can be understood to mean the aforementioned support structure or an actuator or actuators. By way of example, the inner part is clamped with and/or screwed to the support structure. The support structure has a joining point, for example comprising a screwed connection, for joining the inner part. Conversely, it is also possible for the outer part to be connected to the support structure and the inner part to be connected to the optical element.

The connecting parts act as what are known as flexures and allow a movement of the inner part relative to the outer part, or vice versa. In the present case, a “flexure” is generally understood to mean a region, for example a cross-sectional narrowing or thinning, of a component, which region enables a relative movement between two rigid-body regions of the component by bending or torsion. In the present case, the outer part and the inner part can form the rigid body regions, between which the elastically deformable connecting parts are provided as flexures. The connecting parts themselves may additionally have grooves, cross-sectional narrowings or cross-sectional thinnings, which act as flexures provided directly on the connecting parts. This achieves a further improved deformability of the connecting parts.

In the present case, the fact that the connecting parts are “elastically” or “resiliently” deformable means that, in particular, the connecting parts can be brought from a non-deformed state into a deformed state with the aid of a force or a moment. As soon as the force or the moment no longer acts on the respective connecting part, the latter is brought back automatically from the deformed state to the non-deformed state. For example, the connecting parts thus are resiliently deformable.

Each connecting part can have a cross-sectional area that can be shaped as desired. For example, the cross-sectional area is rectangular, triangular, round, cruciform or the like. The connecting parts have a connecting part width and a connecting part height. In the present case, an “aspect ratio” may be understood to mean a ratio of connecting part height to connecting part width. The stiffness of the connecting parts can be modified by changing the aspect ratio. It is desirable for a first connecting part and a second connecting part to be provided. That is to say, precisely two connecting parts may be provided. However, in general there are any desired number of connecting parts.

The cross-sectional area of the connecting parts may be constant viewed along a connecting part length of the connecting parts. In the present case, the “connecting part length” should be understood to mean a length of the respective connecting part along its main direction of extent, along which the connecting part, starting from the inner part, extends towards the outer part. The cross-sectional area may also change viewed along the connecting part length. For example, the cross-sectional area increases from the outer part, as a starting point, to the inner part, or vice versa.

In addition to the connecting parts, the inner part may also be suspended on the outer part with the aid of additional leaf springs. The leaf springs additionally stiffen the mirror socket along the centre axis or third spatial direction and stiffen the mirror socket only minimally in the two other spatial directions. The leaf springs can be folded. For example, each leaf spring has a first leaf spring portion and a second leaf spring portion. The leaf spring portions can be inclined with respect to one another. By way of example, the first leaf spring portion and the second leaf spring portion may be oriented perpendicular to one another.

The connecting parts can extend in the spatial direction of the two spatial directions which provides for the greater stiffness along it. This can be the second spatial direction or y-direction. However, the connecting parts may also extend in the first spatial direction or x-direction. In this latter case, the mirror socket has its greater stiffness viewed in the first spatial direction or x-direction. The mirror socket can have its greatest stiffness viewed in the third spatial direction or z-direction. That is to say, the stiffness of the mirror socket viewed in the third spatial direction or z-direction is greater than in the other two spatial directions.

According to an embodiment, the connecting parts extend in the spatial direction viewed along which the mirror socket has the greater stiffness.

As mentioned previously, this can be the second spatial direction or y-direction. In this case, the connecting parts may extend linearly along this spatial direction. However, the connecting parts may also be curved, for example arcuately curved. Optionally, the connecting parts extend in the second spatial direction or y-direction. Accordingly, the mirror socket can have a lower stiffness viewed perpendicular to the connecting parts than along the connecting parts.

According to an embodiment, the inner part is arranged between a first connecting part and a second connecting part.

There can in general be any desired number of connecting parts. However, it is desirable for exactly two connecting parts to be provided, between which the inner part is placed. The connecting parts can be connected to the inner part with the aid of joining points that act as flexures. By way of example, the connecting parts can be cut free from the outer part with the aid of slots. For example, the slots can be produced with the aid of a wire erosion method.

According to an embodiment, the outer part, the inner part and the connecting parts are connected to one another in integral, for example materially integral, fashion.

Here, “one piece” or “integral” means that, in particular, the outer part, the inner part and the connecting parts form a common component, specifically the mirror socket, and are not put together from different subcomponents. In the present case, “materially integral” means that the outer part, the inner part and the connecting parts are fabricated from the same material throughout. The materially integral embodiment is optional. An implementation with different materials is also possible in general. Optionally, the mirror socket is fabricated from a metallic material. For example, an iron-nickel alloy, for example Invar, can be used. For example, the mirror socket can be produced with the aid of a milling method and/or a wire erosion method. However, the mirror socket can also be produced with the aid of an additive or generative production method, for example with the aid of a 3D printing method.

According to an embodiment, the connecting parts extend parallel to and spaced apart from one another.

As mentioned previously, the connecting parts can extend in the second spatial direction or y-direction. Viewed in the first spatial direction or x-direction, the connecting parts can be placed spaced apart from one another in such a way that the inner part can be placed between the connecting parts. For example, the connecting parts are connected on both end sides to the outer part and are connected centrally to the inner part. According to an embodiment, the connecting parts have arcuate, for example circularly arcuate, curvature.

Thus, the connecting parts can extend around the inner part at least in portions. Thus, the connecting parts can run around or surround the inner part. As a result of the arcuate geometry of the connecting parts, it is possible to increase the connecting part length of the connecting parts in comparison with a straight arrangement of the connecting parts.

According to an embodiment, the connecting parts each have a first connecting part portion and a second connecting part portion, wherein the first connecting part portion and the second connecting part portion are connected to one another with the aid of deflection portions such that the connecting parts have a circumferentially closed geometry.

In this case, the connecting part portions may extend in a straight line and parallel to one another. Alternatively, the connecting part portions may also have arcuate, for example circularly arcuate, curvature. The connecting part portions may extend parallel to one another in this case, too. Together, the connecting part portions and the deflection portions form a circumferentially closed geometry, for example a ring-shaped geometry. For example, the connecting parts are O shaped. Accordingly, the term “ring shaped” also comprises closed geometries that are not circular. Alternatively, the connecting part portions and the deflection portions may also be arranged in such a way that the connecting parts have a circumferentially open geometry. In this case, the connecting parts can be zigzag shaped or have meandering curvature, for example.

According to an embodiment, the connecting parts jointly form a ring connecting part, which runs around the inner part at least in portions.

The inner part is placed within the ring connecting part. The ring connecting part may be circumferentially closed. In this case, the ring connecting part runs completely around the inner part. The ring connecting part may be connected to the inner part with the aid of joining points and to the outer part with the aid of further joining points. The joining points of the inner part and the joining points of the outer part can be placed with an offset of 90° from one another. Alternatively, the ring connecting part may also be circumferentially open. For example, the ring connecting part is connected to the inner part with the aid of exactly one joining point and to the outer part with the aid of two joining points in this case. The ring connecting part can have a circular or else oval shape with principal axes of different length. Accordingly, “ring shaped” does not necessarily mean circular in the present case. Thus, not only the connecting part length but also a curve shape of the connecting parts can be used to adapt the stiffness in the x-direction and in the y-direction. A greater stiffness can be obtained in the direction of the longer principal axis. A lesser stiffness can be obtained across the major principal axis, which is to say along the minor principal axis.

According to an embodiment, the connecting parts have a connecting part height viewed along the centre axis, wherein, starting from the outer part, the connecting part height varies in the direction of the inner part.

In this case, “varies” means that, in particular, the connecting part height changes, for example becomes higher or lower. By way of example, this can be achieved by milling, bevelling or the like. The stiffness of the connecting parts can be adapted as a result. As a result, an installation space-restricting volume can be used efficiently.

According to an embodiment, the mirror socket contains drilled holes, through which a cutting wire can be guided for the purpose of producing the mirror socket.

This can help facilitate the mirror socket producibility.

According to an embodiment, the mirror socket is fabricated from an amagnetic material, for example from molybdenum.

This can enable the use of the mirror socket in magnetic fields as well. Magnetostriction effects are decisive here, as these may lead to a deformation of the material of the mirror socket in the case of a magnetic field change. For example, use can be made of a molybdenum-containing alloy. The term “amagnetic” can be replaced by the term “nonmagnetic”. Alternatively, the mirror socket can also be fabricated from an iron-nickel alloy for example, for example from Invar.

An optical system for a projection exposure apparatus is also proposed. The optical system comprises an optical element, a support structure for carrying the optical element and at least one such mirror socket, wherein the optical element is connected to the support structure with the aid of the at least one mirror socket.

The optical system may comprise any desired number of optical elements and/or mirror sockets. The optical system can be a projection optical unit or a part of such a projection optical unit. Therefore, the optical system can also be referred to as projection optical unit. However, the optical system can also be an illumination system or a part of such an illumination system. Therefore, the optical system can alternatively also be referred to as illumination system. However, the assumption is made below that the optical system is a projection optical unit or part of such a projection optical unit. The optical system is suitable for EUV lithography. However, the optical system can also be suitable for DUV lithography.

As mentioned above, the optical element can be a mirror, for example an EUV mirror. The support structure can be a force frame as mentioned above. In the present case, the support structure “carrying” the optical element means that, in particular, the support structure is able to absorb a weight of the optical element. Thus, for example, a weight of the optical element can be transferred to the support structure via the mirror socket. The mirror socket can have the object of mechanically decoupling the optical element from the support structure such that no parasitic forces, which for example may lead to an unwanted deformation of the optical element, are introduced into the optical element at the mirror socket.

Optionally, a plurality of mirror sockets are assigned to the optical element. The mirror sockets couple the optical element to the support structure. The optical element has six degrees of freedom, specifically three translational degrees of freedom in each case along the first spatial direction or x-direction, the second spatial direction or y-direction, and the third spatial direction or z-direction, and also three rotational degrees of freedom in each case about the three spatial directions. That is to say, a position and an orientation of the optical element can be determined or described with the aid of the six degrees of freedom.

The “position” of the optical element should be understood to mean in particular its coordinates in relation to the x-direction, the y-direction and the z-direction. The “orientation” of the optical element should be understood to mean in particular its tilt in relation to the three spatial directions. That is to say, the optical element can be tilted about the x-direction, the y-direction and/or the z-direction. This gives six degrees of freedom for the position and orientation of the optical element.

A “pose” of the optical system comprises both its position and its orientation. The term “pose” is accordingly replaceable by the wording “position and orientation”, and vice versa. In the present case, an “adjustment” or “alignment” of the optical element is understood to mean in particular a change in the pose of the optical element. Adjusting or aligning the optical element can be implemented in several or all of the six aforementioned degrees of freedom. For example, underlay elements, for example in the form of washers, may be placed under the mirror sockets in order to adjust the pose of the optical element.

According to an embodiment, the optical system also comprises three mirror sockets, wherein the optical element has six degrees of freedom and wherein each mirror socket is assigned exactly two of the degrees of freedom.

For example, each mirror socket has high stiffness in the two degrees of freedom assigned to the respective mirror socket and less stiffness in the four remaining degrees of freedom. However, this is not mandatory. By way of example, it is also possible for one mirror socket to be assigned three degrees of freedom, a further mirror socket to be assigned two degrees of freedom and a further mirror socket to be assigned one degree of freedom. The three mirror sockets can be placed at corners of an imaginary triangle formed by the three mirror sockets.

According to an embodiment, each of the three mirror sockets has a plane spanned by the centre axis and the spatial direction viewed along which the mirror socket has the smaller stiffness, wherein the three mirror sockets are arranged such that the planes intersect one another in a common line of intersection.

For example, the plane is spanned by the centre axis or third spatial direction or z-direction and the first spatial direction or x-direction. Hence, the mirror socket has its greatest stiffness perpendicular to this plane, which is to say viewed in the second spatial direction or y-direction. Optionally, the three mirror sockets are arranged such that their lateral flexible direction in each case points radially in the direction of a centre of the optical element and their laterally stiff direction is oriented perpendicular thereto. For example, the line of intersection lies at the centre of the optical element.

Furthermore, a projection exposure apparatus having such a mirror socket and/or such an optical system is proposed.